1. Field of the Invention
The present invention relates to a method for controlling the temperature of a print head in an ink jet printing apparatus.
2. Description of the Related Art
Ink jet printing apparatuses are based on what is called a non-impact printing scheme and characterized by their ability to print various print media at high speeds and little noise involved in printing. Ink jet printing apparatuses are thus commonly employed as printing mechanisms for various apparatuses such as printers, word processors, facsimile machines, and copiers.
The ink jet printing apparatus is generally composed of an ink jet print head comprising ejection ports from which ink is ejected and a supply system that supplies ink to the print head. A typical scheme for ink ejection uses electrothermal conversion elements (hereinafter referred to as heaters). With this scheme, an electric pulse serving as a print signal is provided to electrothermal conversion elements disposed in a pressurization chamber located inward of the ejection ports, to generate heat. Thermal energy is thus applied to ink, which thus has its phase changed to generate bubbles. The pressure of the bubbles is used to eject print droplets for printing. This scheme makes it possible to easily and accurately manufacture a print element substrate provided densely with a large number of electrothermal conversion elements, wires, and the like, via a manufacturing process similar to that for semiconductors. This in turn enables printing at a higher resolution and a higher speed. As a result, the sizes of printing apparatuses using ink jet print heads can be further reduced.
With the method for ink jet printing, the temperature of ink is a very important parameter in keeping constant the level of stability of ink ejection and the amount of ink ejected. This is because physical properties such as ink viscosity and surface tension vary with temperature, thus varying an ejection state. In particular, in a low temperature environment, the increased ink viscosity may make ejection unstable to degrade print quality. Thus, when an apparatus is used for printing, the ink may be heated to a given temperature before printing is started. To achieve this, heaters for a purpose other than ejection may be provided inside or outside the print head. Further, if ejection is not performed for a given time, moisture unavoidably evaporates from the ink in the vicinity of the ejection ports to increase the ink viscosity, degrading the ejection state. Thus, a recovery operation (preliminary ejection) is performed in which a given amount of ink is ejected during printing or before starting a new printing operation and until normal ejection can be performed in a place other than a print area. When a recovery operation is performed during printing, the printing is suspended, reducing print speed. To minimize the number of recovery operations, the temperature of the ink may be controlled by adjusting the temperature of the head so as to improve the ink ejection state. To control the ink temperature, a heating source such as heat retaining heaters (sub-heaters) is provided on the same substrate with ejection heaters which heat the ink to generate bubbles so that the pressure of the bubbles causes the ink to be ejected. Japanese Patent Laid-Open No. 5-220965 proposes a configuration that directly or indirectly heats ink using the above method. A method for heating ink using ejection heaters involves detecting the temperature of the print head, driving the heaters with a pulse of an appropriate pulse width insufficient for causing bubbling (short pulse) until a predetermined temperature is reached, and stopping energization when the predetermined temperature is reached. A method for heating ink using sub-heaters involves detecting the temperature of the print head, energizing the sub-heaters until a predetermined temperature is reached, stopping energization when the predetermined temperature is reached, and subsequently performing energization again when a certain temperature is reached. On the other hand, Japanese Patent Laid-Open No. 8-58077 proposes a heating arrangement using only a plurality of sub-heaters with different heating values. First, sub-heaters with a larger heating value are used for rapid heating, and a certain time later, sub-heaters with a smaller heating value are used to control the temperature so that the head temperature will not become excessively high while reducing the time required for heating.
However, providing sub-heaters in the print head as is the case with the conventional techniques requires the corresponding space. However, it is difficult to provide such a space in the recent print heads that tend to have densely integrated nozzles. Further, with sub-heaters, temperature control needs to be performed on both ejection heaters and sub-heaters. This complicates the controlling operation itself.
On the other hand, if the ink temperature is controlled using only the ejection heaters without providing any sub-heaters, temperature control must be performed between ink ejections during printing. This not only reduces the print speed but also complicates the controlling operation. The constitution in which the temperature is controlled until immediately before the ink ejecting, and the constitution in which the temperature is not controlled while printing, may be adopted, in order to avoid reducing the print speed and complicating the controlling operation. However, in such a constitution in which the temperature is not controlled while printing, a long print time or a small amount of an image data to be printed may lower the head temperature below the desired value. This prevents the desired ejection state from being established.